The present invention relates to a method of die casting by forcibly injecting molten metal into a mold cavity in a metallic mold assembly, and, more particularly, it relates to improvements in the method of die casting wherein a plunger is driven in a sleeve so as to forcibly inject molten metal poured in the sleeve into the mold cavity at a high pressure by means of a plunger tip secured to the tip of the plunger.
In such a method of die casting, the inner surface of the sleeve and the forward end surface of the plunger tip which are subjected to relative shifting movement at each shot of die casting are rendered to be a high temperature. To avoid the temperature rise, the sleeve and the plunger tip are usually provided with a cooling construction such as internal passages, respectively, through which cooling liquid such as water is circulated so as to maintain them at a low temperature. However, due to difference in the heat capacity between the sleeve and the plunger tip, variation in the clearance between the inner surface of the sleeve and the peripheral surface of the plunger tip can not be avoided when the temperature thereof varies. In general, the influence due to expansion of the plunger tip having a relatively little heat capacity is great and the clearance between the plunger tip and the sleeve immediately after each shot of die casting is reduced and, after a time period during which cooling is effected, the clearance is gradually increased and is restored to the initial condition. Such a variation in the clearance naturally takes place repeatedly as the time lapses in each shot in which the molten metal is injected into the mold cavity.
The clearance between the inner surface of the sleeve and the peripheral surface of the plunger tip is an important factor affecting the relative shifting movement therebetween. Particularly, if the plunger tip is retracted under the condition that the clearance is reduced to a value not acceptable for proper die casting, the failure or breakage will take place in the die casting apparatus such as the sleeve and the plunger tip due to the biting of casting fins or flashes between the inner surface of the sleeve and the peripheral surface of the plunger tip.
In order to prevent such a trouble caused by the variation in the clearance between the inner surface of the sleeve and the peripheral surface of the plunger tip, the control of the operation of the plunger has heretofore been effected in general by estimating the value of the clearance according to the lapse of time utilized as a factor for determining the same, and initiating the operation of the plunger on the basis of the estimated clearance. In other words, in order to avoid the biting of the fins or the flashes between the inner surface of the sleeve and the peripheral surface of the plunger tip caused due to too small clearance in each shot cycle wherein the molten metal is forcibly injected into the mold cavity by the forward movement of the plunger, and the plunger is retracted after the solidified die cast product has been taken out of the mold cavity so as to be ready for the succeeding die casting operation, the control of the operation of the plunger has been effected hereinbefore in such a manner that the initiation of retracting of the plunger is delayed until the time period lapses in which it is assumed to be sufficient to restore the required value of clearance.
With such a control of operation based on the time chart of the variation in the clearance, however, an excessive unacceptable clearance or even a condition that the plunger tip is rendered to be press-fitted in the sleeve might take place, should a failure take place in the cooling system or an abnormal condition of the die casting apparatus caused by the fluctuation in the respective shot cycle, even though a sufficient time period be given for insuring the security or safety of the apparatus. That is, the above described control of operation of the plunger effected on the basis of the time chart has a fatal disadvantage in that it can not insure sufficient clearance for achieving security in case of occurrence of an abnormal condition of operation.
In addition to the above, the efficiency in the die casting operation is greatly affected depending upon the set time period of initiating the operation of the plunger after each shot. The higher the security is set, the more the efficiency is deteriorated, thereby resulting in disadvantage rendering the improvement in efficiency in the die casting to be difficult.
Under such circumstances, it has long been desired to make it possible to detect or ascertain positively and efficiently the value of the clearance even though an abnormal condition occurs in the die casting apparatus so as to avoid the failure or breakage of the die casting apparatus.